Recently, according to the trend toward the use of light parts and thin parts in the field of display and electronic industries, research on a flexible base material, particularly, a plastic substrate, has been actively conducted. When a substrate is fabricated using plastic, which is a thin, light and flexible material, the substrate has advantages such as high durability, low price and a simple fabrication process. It is expected that a display manufactured using a flexible substrate including this plastic substrate can be implemented as a paper-like display through a flexible display or a rollable display, in the development of the display industry.
However, since a flexible substrate, particularly a plastic substrate, which will replace a glass substrate used to manufacture a Liquid Crystal Display (LCD) panel or an Organic Light Emitting Display (OLED) panel, is flexible, there is a problem in that it is difficult to transport the substrate in a process of manufacturing a panel using the glass substrate.
To overcome this problem, a method of attaching the flexible substrate to a transporting support formed of a relatively thick plate glass or plastic plate, transporting it during a manufacturing process, and finally detaching it from the support has been commonly known. This method includes attaching a plastic substrate for a liquid crystal display device to a thick plate glass or plastic plate, which is a flat transporting support, performing resist-forming, etching and cleaning processes directly using a process and apparatus for manufacturing a liquid crystal display device using the commonly used glass substrate, and finally separating the plastic substrate from the support.
A double-sided adhesive tape, one side of which has a low adhesive strength and the other side of which has a relatively high adhesive strength, or a release sheet, the adhesive strength of which is easily controlled by temperature or light, has been proposed as a general adhesive for use in the above method. When this adhesive is used, there are advantages in that panel manufacturing lines using the commonly used glass substrate can be directly used without alteration, so that the investment cost of equipment for manufacturing a panel using the flexible substrate is minimized, with the result that the panel can be manufactured at low cost. Technologies related thereto are as follows.
Japanese Unexamined Patent Publication Nos. Heisei08-53156, 2000-252342 and 2002-258252 disclose temperature sensitive adhesives which have high adhesive strength at the time of the transport and processing of electronic parts and facilitate the peeling of the electronic parts due to the decrease of adhesive strength after the transport and processing thereof.
Specifically, Japanese Unexamined Patent Publication No. Heisei08-53156 relates to an adhesive using a (meth)acrylic acid polymer having side-chain crystallinity, in which the carbon number of an ester is 8 or more. This cold-peelable adhesive has an adhesive strength ranging from 100 to 800 N/m (250 to 2000 gf/25 mm) at a temperature not lower than the melting point, and has an adhesive strength of 100 N/m (250 gf/25 mm) or less at a temperature not higher than the melting point. Therefore, a substrate cannot be detached in a high-temperature process, but can be easily detached at a temperature not higher than the melting point (−10° C.). However, since the adhesive is an adhesive containing an acrylic resin as a principal component, the property of the adhesive is easily changed, the chemical resistance thereof is decreased, and bubbles are generated at a temperature of 150° C. or more in a Thin Film Transistor (TFT) process and/or a color filter process, which are substrate manufacturing processes for manufacturing an LCD panel or an OLED panel, therefore there is a problem in that a lithography process cannot be performed due to the low dimensional stability of flexible substrate.
Japanese Unexamined Patent Publication No. 2000-252342 discloses a method of applying a double-sided adhesive tape one side of which is coated with a warm-peelable adhesive and the other side of which is coated with an adhesive for bonding a support plate, like a conventional adhesive. The warm-peelable adhesive has an advantage in that, when the warm-peelable adhesive is heated at a temperature of 100° C. for 1 minute, the adhesion area is decreased due to foaming, so that the adhesive strength is decreased, thereby the peeling-off adhesive is easily peeled off from the flexible substrate. However, as described above, there has been a problem in that the warm-peelable adhesive cannot be applied to a process of manufacturing an LCD panel or an OLED panel at high temperatures due to foaming at temperatures of 100° C. or higher.
Japanese Unexamined Patent Publication No. 2002-258252 discloses a method of using a temperature sensitive adhesive the adhesive strength of which varies according to a reversible change between a crystal state and an amorphous state. However, since this adhesive is also an acrylic adhesive, there are the same problems as before, that is, low heat resistance, low chemical resistance, partial foaming or partial peeling at high temperatures.
Meanwhile, Japanese Unexamined Patent Publication No. Heisei08-086993 discloses a jig for transporting a flexible substrate, which is provided with a support that includes a tacky adhesive layer. That is, disclosed is a method of decreasing stress by forming an adhesive layer or an intermediate layer between the support of the jig and the tacky adhesive layer to prevent bubbles or peeling between the substrate and the tacky adhesive layer resulting from the stress generated by the difference in the thermal expansion coefficients. This method has an advantage in that the adhesive strength between the support and the tacky adhesive layer is increased due to the adhesive layer, thereby enabling easy removal of the flexible substrate in the final process. Although it is described in this patent document that a silicon rubber having better heat-resistance than that of an acrylic resin is used as the tacky adhesive, and a jig provided with a tacky adhesive can be used several times, there have been problems in that, after the tacky adhesive has been used several times, the adhesive strength of the tacky adhesive is decreased because extraneous materials adhere to the surface of the tacky adhesive, and the removal of an adhesive to reuse the support cannot be easily conducted.
Accordingly, a disposable adhesive for transporting a flexible substrate, which has advantages in that bubbles and partial peeling do not occur, that is, heat resistance, chemical resistance and dimensional stability are excellent even in a high temperature process or an acid and base process, such as a resist forming process, an etching process and a cleaning process, which are processes for manufacturing an LCD panel or an OLED panel comprising a flexible substrate, particularly, a plastic substrate, by directly utilizing conventional panel manufacturing apparatuses and processes using a glass substrate, the flexible substrate is easily detached, and the adhesive is easily removed from the glass substrate, which functions as a support, is keenly required.